DE102011081627A1 - Porous medium with increased hydrophobicity and method for producing the same - Google Patents

Abstract

The present invention provides a porous medium with increased hydrophobicity and a method of manufacturing the same, wherein by forming nano-projections having a large aspect ratio by performing plasma etching on the surface of a porous medium having a surface roughness in the micrometer range, a micro-nano double structure is provided and a hydrophobic thin layer is deposited on the surface of the micro-nano double structure, so that the hydrophobicity is significantly increased. When this highly hydrophobic porous medium is used as a gas diffusion layer in a fuel cell, it is possible to efficiently remove the water generated during the electrochemical reaction of the fuel cell, so that the fuel cell can be prevented from flooding. In addition, it is possible to supply the reaction gases such as hydrogen and air (oxygen) to a sufficient amount of a membrane electrode assembly (MEA) so that the performance of the fuel cell is improved.

Description

background

(a) Technical area

The present invention relates to a hydrophobic porous medium and a method for producing the same. More specifically, it relates to a highly hydrophobic porous medium and a method for producing the same.

(b) Prior art

An electrochemical reaction in a polymer electrolyte membrane fuel cell (PEMFC) to generate electricity proceeds as follows. Hydrogen supplied to an anode ("oxidation electrode") in a membrane electrode assembly (MEA) of the fuel cell is decomposed into hydrogen ions (protons, H ⁺ ) and electrons (e ^- ). The hydrogen ions are passed through a polymer electrolyte membrane to a cathode ("reduction electrode"), and the electrons are passed through an external circuit to the cathode, so that electricity is generated by the flow of the electrons.

Further, at the cathode, the oxygen molecules, the protons and the electrons react with each other, generating electricity and heat and, at the same time, water as a reaction by-product.

The range defining the electrochemical performance of the fuel cell is generally divided into three sections: (i) an "activation loss" section due to the loss of electrochemical reaction kinetics; (ii) an "ohmic loss" portion due to the contact resistance at the interfaces between the respective components and a loss of ion conduction in the polymer electrolyte membrane; and (iii) a section of "mass transport loss" or "loss of concentration" due to restrictions of mass transport of the reaction gases [ R. O Hayre, S. Cha, W. Colella, FB Prince, Fuel Cell Fundamentals, Chapter 1, John Wiley & Sons, New York (2006) ].

When there is a sufficient amount of water generated during the electrochemical reaction, this is preferably to maintain the moisture in the polymer electrolyte membrane. However, if an excess amount of generated water is not appropriately removed, "flooding" occurs at a high current density, thereby preventing the reaction gases from being effectively conducted to the fuel cell and increasing the voltage loss [ MM Saleh, T. Okajima, M. Hayase, F. Kitamura, T. Ohsaka, J. Power Sources, 167, 503 (2007) ].

A typical porous medium constituting the fuel cell is a gas diffusion layer (GDL) having a structure in which a microporous layer (MPL) and a macroporous substrate or a macroporous sublayer are combined with each other.

Commercially available gas diffusion layers have a bilayer structure comprising a microporous layer having a pore size of less than 1 micron, as measured by the penetration depth of mercury, and a macroporous substrate or macroporous sublayer having a pore size of 1 to 300 microns [ XL Wang, HM Zhang, JL Zhang, HF Xu, ZQ Tian, J. Chen, HX Zhong, YM Liang and BL Yi, Electrochimica Acta, 51, 4909 (2006) ].

The gas diffusion layer is attached to the outside of catalyst layers for the anode and the cathode, which are applied to the two faces of the polymer electrolyte membrane in the fuel cell. The gas diffusion layer effects supply of the reaction gases such as hydrogen and air, transfer of the electrons generated by the electrochemical reaction, and discharge of the water generated by the reaction to minimize flooding hazard of the fuel cell. L. Cindrella, AM Kannan, JF Lin, K. Saminathan, Y. Ho, CW Lin, J. Wertz, J. Power Sources, 194, 146 (200) ; XL Wang, HM Zhang, JL Zhang, HF Xu., ZQ Tian, J. Chen, HX Zhong, YM Liang, BL Yi, Electrochim. Acta, 51, 4909 (2006) ].

In particular, in order to increase mass transport and maintain the high performance of the cell by efficiently removing the water generated during the electrochemical reaction of the fuel cell, it is very important to impart hydrophobicity to the microporous layer and the macroporous substrate by suitably hydrophobic Means, such as Polytetrafluoroethylene (PTFE) is introduced into this [ S. Park, J. -W. Lee, BN Popov, J. Power Sources, 177, 457 (2008) ; G. -G Park, Y. -J. Son, T. -H. Yang, Y. -G. Yoon, W.-Y. Lee, C. -S. Kim, J. Power Sources, 131, 182 (2004) ].

Conventionally, however, a wet chemical method of imparting hydrophobicity has been used, and therefore the manufacturing process itself is complicated and it is difficult to evenly distribute the hydrophobic agent such as PTFE on the gas diffusion layer.

Moreover, according to the conventional method for producing the gas diffusion layer, it is difficult to impart high hydrophobicity or superhydrophobicity to a porous medium which has already been subjected to impregnation treatment, which corresponds to a contact angle (static constant angle) of 150 ° or more.

In conventional studies, various attempts have been made to impart hydrophilicity to the surface of the porous medium by using various plasma techniques, such as using oxygen, nitrogen, ammonia, silane (siH ₄ ), organometallics, etc. [DR Mekala, DW Stegink, MM David, JW Frisk, US 2005/0064275 A1 (2005); Korean Patent Publication No. 10-2006-0090668 (2006)], which differs from the object of the present invention to impart the porous medium, a high hydrophobicity.

In addition, attempts have been made during the manufacture of the electrodes of the MEA to employ surface treatment techniques using plasma [GH Nam, SI Han, Korean Patent Publication No. 10-2009-0055301 (200); MG Min, GS Chae, SG Kang, Korean Patent No. 10-0839372 (2008); WM Lee, IG Goo, JH Sim, Korean Patent No. 10-0681169 (2007); HT Kim, HJ Kwon, Korean Patent No. 10-0599799 (2006)], but relate to a process for forming a catalyst layer containing a catalyst and a binder. That is, these methods serve to chemically generate a hydrophilic or hydrophobic surface by modifying the surface of the catalyst layer using plasma techniques, whereby it is limitedly possible with these methods to form high hydrophobicity on the surface of the porous medium ,

The information provided in this Background section is merely to help understand the background of the invention, and thus may include information that is not prior art, as is already known to one of ordinary skill in the art in this country.

Summary of the Revelation

The present invention provides a method for increasing the hydrophobicity on the surface of a porous medium which can be effectively used in a fuel cell.

In one aspect, the present invention provides a porous medium having increased hydrophobicity. This porous medium has a micro-nano-double structure in which nanometer-sized protrusions or collapsed pores are formed on the surface of the porous medium having a surface roughness in the micrometer range, and a hydrophobic thin layer formed on the surface of the micropores. Nano-double structure is deposited on.

In a further aspect, the present invention provides a method of making a porous medium having increased hydrophobicity. This method involves providing a porous medium having a surface roughness in the micrometer range; forming a micro-nano-double structure on the surface of the porous medium by generating nanometer-sized protrusions or collapsed pores by plasma etching; and depositing a hydrophobic thin layer on the surface of the micro-nano-double structure.

In the following, further aspects and preferred embodiments of the invention are explained.

Brief description of the figures

The foregoing and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof, which are illustrated in the accompanying drawings, which are given by way of illustration only and are not intended to limit the invention in any way. In the figures:

1 Fig. 12 is a schematic diagram showing a micro-nanocomposite structure according to the illustrative embodiment of the present invention formed by performing plasma etching on a porous medium;

2A to 2D FIG. 13 is an SEM photograph showing the surface of a microporous layer of a gas diffusion layer before and after oxygen plasma etching and "oxygen plasma etching by means of oxygen + deposition of a hydrophobic thin layer" according to an illustrative embodiment of the present invention;

3 Fig. 12 is a graph showing the change in the contact angle of one drop on the surface of a microporous layer while changing the duration of the plasma etching by means of oxygen;

4A and 4B FIG. 13 is an SEM photograph showing the surface of a macroporous substrate before and after plasma etching by oxygen according to an illustrative embodiment of the present invention; FIG. and

5 FIG. 12 is a graph showing the change in the contact angle of a droplet on the surface of a macroporous substrate while changing the duration of plasma etching by means of oxygen. FIG.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified illustration of various preferred features illustrative of the basic principles of the invention. Concrete features of the embodiment of the present invention as disclosed herein, including, for example, particular dimensions, orientations, locations, and shapes, will be determined in part by the particulars of intended use and use.

In the figures, the reference numerals in all figures of the drawing indicate the same or equivalent parts of the present invention.

Detailed description

Reference will now be made in detail to various embodiments of the present invention, which is illustrated by way of example in the accompanying drawings and described below. While the invention is described in conjunction with exemplary embodiments, it is to be understood that the present description is not intended to limit the invention to these exemplary embodiments. Rather, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents, and other embodiments included within the true spirit and scope of the invention as defined in the appended claims.

It should be understood that the term "vehicle" or "vehicle" or any other similar term as used herein includes motor vehicles generally, such as passenger cars, including SUVs, buses, trucks, various Commercial vehicles, watercraft, including a variety of boats and ships, aircraft and the like, as well as hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-fueled vehicles, and vehicles operated with other alternative fuels (e.g., fuels that come from sources other than petroleum). As used herein, a hybrid vehicle is a vehicle that can be operated in two or more ways, for example, a vehicle that can run on both gasoline and electricity.

The foregoing and other features of the invention are discussed below.

The present invention provides a porous medium (PM) used in a fuel cell and a method for producing the same, the porous medium having a surface having an increased hydrophobicity.

In particular, the highly hydrophobic porous medium of the present invention has a surface having a micro-nano-double structure in which a surface roughness on the surface of the porous medium having a surface roughness in the micrometer range (a macroporous substrate which will be described later has a surface roughness in the micrometer range) nanometer-sized protrusions or nano-protrusions ("nanoprotrusions") or collapsed nanopores have been generated while forming a hydrophobic thin layer the surface of the micro-nano-double structure was deposited, whereby the hydrophobicity due to the micro-nano-double structure and the hydrophobic thin layer is increased.

In the following, the micro-nano-double structure represents a composite structure having microstructures and nanostructures in which artificially nanometer-sized protrusions or collapsed nanopores were formed on the surface of the porous medium having an intrinsic roughness in the micrometer range by means of a plasma etching treatment.

Since the macroporous substrate has a surface roughness in the micrometer range, the micrometer-sized protrusions or collapsed pores on the surface thereof together with the artificially generated nanoprints or nanopores form the micro-nano-double structure.

Besides, the carbon particles of the microporous layer also have a fine surface roughness, so that when nano-protrusions or nanopores are formed on the microporous layer by means of a plasma etching treatment, the nanostructures produced by the plasma etching treatment form a double structure on the microporous layer together with the carbon particles.

The surface roughness is predetermined by the nano-protrusions or collapsed nanopores formed on the surface of the material for the porous medium (corresponding to the carbon particles of the microporous layer or the carbon fibers of the macroporous substrate).

As a result, the highly hydrophobic porous medium having the micro-nano-double structure of the present invention has the characteristics that the wettability on both (outer) surfaces is substantially lower than that of conventional porous media, and that the contact angle (ie, the static Contact angle) of a fluid such as pure water on the surface of the porous medium is 150 ° or more, which will be described in detail below.

In order to overcome the limitations of achieving high hydrophobicity in a conventional method of producing the porous medium, the surface of the porous medium such as the gas diffusion layer (ie, the surface of the microporous layer and the surface of the macroporous substrate) becomes high in both hydrophobicity and structural properties as well as by chemical modifications during the manufacturing process of the present invention, wherein a method of optimizing the nanostructures with a high aspect ratio on the surface of the porous medium, a method of structurally modifying the surface with a micro-nano-double roughness structure and a method of chemical Modification for forming a chemical-hydrophobic surface by depositing a hydrophobic thin layer in combination can be performed.

In experiments, it was found that when the porous medium is treated with dry plasma (ie, plasma etching treatment is performed), nano-protrusions or nanopores are formed (by the plasma etching treatment) and the surface of the porous medium having a surface roughness in the micrometer range is formed Micro-nano-double structure can be combined, and if a hydrophobic thin layer of carbon on the surface of the micro-nano-double structure, for example by means of plasma deposition, is formed, the hydrophobicity of the porous medium can be substantially increased. The thus-formed porous medium having enhanced hydrophobicity can be effectively used as a gas diffusion layer of a fuel cell and used efficiently for removing the water generated during the electrochemical reaction of the fuel cell.

During the method of modifying the surface of the porous medium by plasma treatment according to the present invention, the nanostructures are formed by etching the surface of the porous medium using, for example, argon (Ar) or oxygen plasma, to provide a structure having the contact surface with one Fluid, such as water, and the hydrophobic thin layer (eg, a hydrophobic thin layer of carbon) is deposited on the surface of the resulting structure. In this case, it is possible to impart high hydrophobicity or superhydrophobicity corresponding to a contact angle of 150 ° or more with respect to a fluid such as pure water. That is, in the treatment with dry plasma, structural and chemical modifications on the surface of the porous medium are possible, and therefore it is possible to easily impart high hydrophobicity suitable for a fuel cell.

A better understanding of increased hydrophobicity on the surface of the porous medium can be achieved by understanding the mechanisms of high hydrophobicity or superhydrophobicity on a solid surface, which is described below.

The hydrophobicity of a solid surface depends on the chemical properties of the solid surface, but when a fine pattern is formed on the solid surface, the hydrophobicity increases significantly, so that the solid surface obtains superhydrophobicity. For example, the contact angle of the surface having a fine structure with protrusions or pores with respect to water having values in a range of 150 ° to 170 ° is relatively large, so that compared to a flat surface which has undergone the same chemical treatment , a super hydrophobicity is imparted.

At the same time, a surface having a structure with protrusions or pores may have a self-cleaning function that allows a drop to be easily removed on the solid surface under conditions in which the hysteresis of the contact angle is reduced to less than 10 °. Therefore, in order to produce a highly hydrophobic or superhydrophobic surface, a low surface energy surface layer should be formed, and at the same time, the surface layer should have a physical / structural surface roughness.

Regarding the surface roughness, the size distribution of the fine protrusions or pores plays a very important role, and a surface roughness having collapsed pores also exhibits the same properties as a surface roughness having fine protrusions. In particular, when the chemical composition of the surface is controlled and at the same time nanometer-sized pores and micrometer-sized pores are coexistent, a hydrophobic surface and further a superhydrophobic surface can be achieved. Therefore, in the present invention, the intended high hydrophobicity is achieved by carrying out a hydrophobicity enhancing mechanism, which can be obtained by combining the above-described structure and the chemical properties on the surface of the porous medium.

That is, the highly hydrophobic surface is achieved by forming a nanopattern by plasma etching and forming a hydrophobic thin film by plasma deposition on the surfaces of the microporous layer and the macroporous substrate constituting the porous medium such as the gas diffusion layer of a fuel cell , Furthermore, it is possible to impart high hydrophobicity to the surface of the porous medium by simultaneously structurally and chemically controlling the surface properties of the porous medium.

Next, the present invention will be described in more detail with reference to the figures. 1 Fig. 12 is a schematic diagram showing a micro-nanocomposite structure formed by performing plasma etching on a porous medium according to the present invention. It means that 1 schematically shows a microporous layer and a macroporous substrate, which form the gas diffusion layer as a typical example of a porous medium in a fuel cell, and further shows a method of modifying the surface of the porous medium according to the present invention.

The highly hydrophobic porous medium of the present invention has high aspect ratio nanostructures and a hydrophobic thin layer respectively provided on the surface of the microporous layer and the surface of the macroporous substrate.

As in 1 5, the plasma etching treatment for the micro-nano-double structure is carried out respectively on the surface of the microporous layer and on the surface of the macroporous substrate to form nanostructures having a high aspect ratio, and a hydrophobic thin layer is formed on the micro-nano via plasma deposition Formed double structure, whereby a porous medium is produced with a highly hydrophobic surface.

The microporous layer and the macroporous substrate, which form the gas diffusion layer as a porous medium in the fuel cell, are made of the following materials.

Usually, the microporous layer may be formed by preparing a mixture of carbon black powder such as acetylene black, pearl black, etc., and a hydrophobic agent such as polytetrafluoroethylene (PTFE) or fluorinated ethylene propylene (FEP) and the mixture on one or both sides the macroporous layer is applied. The macroporous substrate is generally composed of carbon fibers and a hydrophobic agent such as polytetrafluoroethylene or fluorinated ethylene propylene C. Lim and CY Wang, Electrochim. Acta 49, 4149 (2004) ], and depending on the physical structure, can be largely classified into a felt of carbon fibers, paper of carbon fibers and a cloth of carbon fibers [ S. Escribano, J. Blachot, J. Etheve, A. Morin, R. Mosdale, J. Power Sources, 156, 8 (2006) ; MF Mathias, J. Roth, J. Fleming, and W. Lehnert, Handbook of Fuel Cells Fundamentals, Technology and Applications, Volume 3, Chapter 42, John Wiley & Sons (2003) ].

In the present invention, the plasma etching is performed on each surface of the porous medium having the above-described structure in which the microporous layer and the macroporous substrate are stacked on each other ( 1 ), wherein a pattern of nano-protrusions or nano-pores is formed on the surface of the carbon materials (such as carbon particles and carbon fibers) forming the surfaces of the microporous layer and the macroporous substrate (ie, the surface of the porous medium).

In an illustrative embodiment, preferably by plasma etching, a pattern of nano-protrusions or nano-pores having a width of 1 to 100 nanometers, a length of 1 to 1000 nanometers and an aspect ratio of 1 to 10 on the surface of the carbon materials on the surfaces of the microporous Layer and the macroporous substrate are formed. When the aspect ratio is smaller than 1, the effect of surface roughness can not be fully achieved, whereas when the aspect ratio is larger than 10, the structure of the nanopattern is not stably maintained.

In the microporous layer, the carbon particles, which are not uniform in size, form aggregates and are therefore present in a range of several tens of nanometers to several micrometers. When the plasma etching treatment is performed on the microporous layer, the surfaces of the spherical carbon particles are etched, thereby forming sharp-edged carbon particles having a width of several nanometers, and the hydrophobic thin layer is deposited on the surface of the resulting microporous layer.

Besides, by plasma etching, nano-protrusions or nanopores having a width of 10 to 30 nanometers, a length of 10 to 200 nanometers and an aspect ratio of 1 to 7 are preferable on the surface of carbon fibers of the macroporous substrate having a diameter of 5 to 20 microns train. These nano-protrusions form a nano-pattern with a high aspect ratio, thereby providing the micro-nano-double structure. Furthermore, the surface with the micro-nano-double structure has superhydrophobic and self-cleaning properties.

The hydrophobic thin layer for increasing the hydrophobicity may be a thin hydrocarbon layer containing silicon (Si) and oxygen, or a hydrocarbon thin layer containing fluorine (F), and the hydrophobic thin layer may have a thickness in a range of 0. 1 to 90 nanometers. If the thickness of the hydrophobic thin layer is less than 0.1 nanometer, the effect of increasing the hydrophobicity of the porous medium can not be obtained, whereas if the thickness is more than 90 nanometers, the electrical resistance may increase considerably if the porous one Medium is used as a gas diffusion layer. It is therefore preferred that the hydrophobic thin layer has a thickness in a range of 0.1 to 90 nanometers.

In particular, for using the porous medium as a gas diffusion layer of a fuel cell, it is necessary to suitably adjust the thickness of the hydrophobic thin layer so that the pores are not clogged without increasing the initial electric resistance of the gas diffusion layer.

The silicon and oxygen-containing hydrocarbon material can be precipitated using hexamethyldisiloxane (HMDSO) as a precursor, and the hydrophobicity can be controlled by properly mixing hexamethyldisiloxane and argon gas (not more than 30% by volume).

The contact angle on the surfaces of the microporous layer and the macroporous substrate in the porous medium, the hydrophobicity of which is increased by the plasma treatment described above (to form the nanostructures by plasma etching and to form the hydrophobic thin layer by plasma deposition) is 150 ° or more. Since the nanometer-sized pattern and the micrometer-sized pattern are mixed with the PTFE as the hydrophobic polymer on the surfaces of the microporous layer and the macroporous substrate constituting the porous medium, the contact angle is about 135 to 145 °.

In the porous medium of the present invention, the size of the large aspect ratio nanopattern formed by plasma etching is considerably smaller than that of the conventional surface described above, and the surface roughness is much larger than that of the conventional surface. Therefore, the porous medium of the present invention may have a contact angle (ie, static contact angle) of 150 ° or more, and have highly hydrophobic surface properties due to the micro-nano-double structure. In particular, even a surface containing no PTFE can have high hydrophobicity, which corresponds to a contact angle of 150 ° or more. Besides, the hydrophobic thin film is uniformly deposited on the surface, so that the surface energy is generally low, and thus a highly uniform hydrophobic surface can be obtained.

It is difficult to uniformly introduce a hydrophobic agent such as PTFE incorporated in the microporous layer and the macroporous substrate of a commercially available gas diffusion layer into the surface and the inside, and a complicated wet process must be used for production. Besides, it is difficult to increase the contact angle of the surface of the porous medium to more than 150 °.

The method for producing the highly hydrophobic porous medium according to the present invention comprises: (a) providing a porous medium having only a macroporous substrate or having a structure in which a macroporous substrate and a microporous layer are combined with each other; (b) forming nanostructures in the form of nano-protrusions having a high aspect ratio or collapsed nanopores on the surface of the carbon materials of the porous medium; and (c) depositing a hydrophobic thin layer on the surface of the porous medium on which the nanostructures have been formed.

As in 1 is shown, step (a) serves to provide a porous medium having only a macroporous substrate or having a structure in which a macroporous substrate and a microporous layer are combined with each other, and this method of producing the porous medium is disclosed in US Pat generally known to science. In a preferred embodiment, a commercially available porous medium for the gas diffusion layer in which the microporous layer and the macroporous substrate are combined with each other can be used.

Step (b) serves to form nanostructures with a high aspect ratio by performing plasma etching on both sides of the porous medium comprising the microporous layer and the macroporous substrate on which the nanometer-sized and micrometer-sized surfaces are formed. The plasma etching may be plasma-enhanced chemical vapor deposition (PECVD) or plasma-enhanced chemical vapor deposition (PACVD) and using O ₂ , Ar, N ₂ , He, CF ₄ , CHF ₃ , C ₂ F ₆ , HF or SiF ₄ expire. In addition to the chemical vapor deposition (CVD), the etching can also be carried out by means of a combination of ion beam, chemical vapor deposition by means of a hybrid plasma (hybrid plasma chemical vapor deposition, HPVCD) and atmospheric plasma.

When the surface etched by the plasma etching is increased, it can be seen that a large number of nano-protrusions having a large aspect ratio have been formed, and the change in the distribution of the nano-protrusions on the surfaces of the microporous layer and the macroporous substrate is insufficient the 2 and 4 to recognize.

The 2A and 2 B are scanning electron microscope (SEM) images showing the surface of the microporous layer before plasma etching with oxygen, and the 2C is an SEM image showing the oxygen plasma etched surface of the microporous layer. Next to it is 2D a SEM image showing the surface on which the hydrophobic thin film was finally deposited on the oxygen plasma etched microporous layer.

Compared to the 2A and 2 B before the plasma etching treatment can be, as in 2C 12, it can be seen that the size of the nanopattern is relatively reduced from about 50 nanometers to about 10 to 30 nanometers and the surface is roughened more. The oxygen plasma reacts with the carbon materials by etching the carbon particles on the surface of the microporous layer and the carbon fibers on the surface of the macroporous substrate. Here, the carbon materials are combined with the oxygen plasma to form CO ₂ or CO, whereby the surface is etched.

In the plasma etching method of the present invention, the size and shape of the nanostructures having a large aspect ratio can be controlled by adjusting at least one of etching pressure, accelerating voltage and etching time (ie, the duration of treatment with the plasma). Preferably, the etching pressure is 1 Pa to 10 Pa, and the acceleration voltage is -100 Vb to -1,000 Vb.

If the etching pressure is less than 1 Pa, the formation speed of the surface roughness pattern is too small to effectively form a pattern, whereas if the pressure is larger than 10 Pa, the formation speed of the surface roughness pattern is too large to form a stable pattern.

Besides, when the acceleration voltage is smaller than -100 Vb, the plasma can not be efficiently generated, whereas when the acceleration voltage is larger than -1000 Vb, the process of plasma generation can not be stably maintained.

In addition, since the formation of nanostructures on the surface may have a significant effect on the performance of the fuel cell depending on the duration of the plasma etching, it is important to carry out the plasma irradiation for an optimally long period of time, and therefore the plasma etching is preferably performed over a period of 0.1 to 60 minutes.

If the duration of the plasma etching is less than 0.1 minute, the etching effect is too small to allow development of the nanostructures, whereas it is difficult to control the surface shape of the desired nanostructures for more than 60 minutes due to excessive etching. Furthermore, the total duration of the surface treatment is too long and thus the productivity is reduced.

Step (e) serves to deposit a hydrophobic thin layer on the surface of the porous medium on which a composite pore structure comprising micropores and nanopores has been formed. In order to deposit the hydrophobic thin layer, a mixed gas of argon (in a range of 0 to 30% by volume) and hexamethyldisiloxane or hexamethyldisiloxangas may be used.

The surface properties of the hydrophobic thin layer to increase hydrophobicity depend on the radio frequency (RF) power supply and the amount of argon in a PECVD apparatus.

Therefore, if the RF power supply and the content of argon in the precursor gas are suitably controlled, it is possible to control the hydrophobic properties and obtain an improved thin film.

Hereinafter, examples of the present invention will be described in detail with reference to the drawings.

A method for producing a highly hydrophobic porous medium on the surface of the microporous layer and the macroporous substrate constituting the gas diffusion layer as a typical example of the porous medium in a fuel cell is described in Examples 1 and 2 given below, wherein the present invention but not limited thereto.

Example 1: Material for a microporous layer with increased hydrophobicity

First, a commercially available material for a gas diffusion layer comprising a microporous layer containing carbon powder having a diameter of 10 to 300 nanometers and PTFE, and a macroporous substrate containing carbon fibers in the form of a felt and PTFE was used. However, for the present invention, a material for a gas diffusion layer having a macroporous substrate containing only carbon fibers and not a hydrophobic material such as PTFE may be used. The size of the carbon particles constituting the microporous layer in the material for the gas diffusion layer is not uniform and is in a range of 10 to 300 nanometers.

The basic characteristics and characteristics of the gas diffusion layer used in this example of the present invention are shown in Table 1. The total thickness of the porous medium (i.e., the gas diffusion layer) was obtained by measuring at least 20 times using a Mitutoyo thickness gauge (Mitutoyo Co., Japan).

The weight per unit area of the porous medium was obtained by measuring at least 20 times with a digital balance (A & D Company, Japan), and the air permeability of the porous medium was at least 5 times under conditions of 0.3 kPa using the Gurley method measured. [Table 1] Thickness of the GDL (μm) Type of macroporous substrate Presence or absence of the microporous layer Weight per unit area of GDL (g / m ² ) Air permeability [cm ³ / (cm ² .s)] 426 ± 10 Felt made of carbon fibers available 135 ± 2 3.2 ± 1.6

The 2A and 2 B are little and greatly magnified images of the surface of the microporous layer in this example before plasma etching with oxygen.

The surface of a generated microporous layer was subjected to plasma etching treatment with oxygen using RF-PECVD, and oxygen plasma etching was conducted under conditions using only oxygen as the gas, the etching pressure was 10 Pa, and the RF voltage. 100 Vb to -800 Vb.

The oxygen plasma reacts with etching of the carbon particles on the surface of the microporous layer with the carbon materials, and thereby the carbon materials, forming CO ₂ or CO are connected to the oxygen plasma, whereby the surface of the microporous layer is etched.

Specifically, the surfaces of spherical particles having a size of several tens to several hundreds of nanometers, which form the microporous layer, are etched to form sharp-edged carbon particles having a width of 10 to 20 nanometers.

3 FIG. 12 is a graph showing the change in the contact angle of one drop on the surface of a microporous layer while changing the duration of the plasma etching by means of oxygen. As in 3 is shown, when the duration of the plasma etching with oxygen at a voltage of -400 Vb during the plasma etching is changed to 1, 2, 5 to 10 minutes, the width of the nanoparticles on the surface is reduced to 10 to 20 nanometers, and the The shape of the nanoparticles is changed to nano-projections with a length of 10 to 50 nanometers. As a result, the carbon nanoparticles are changed to nano-projections, and thus the surface roughness increases.

Then, a hydrophobic thin film containing silicon and oxygen was deposited on the surface of the carbon nanoparticles thus formed to increase the hydrophobicity.

The hydrophobic layer was deposited from HMDSO at 13.56 MHz using an RF PECVD with the argon gas content in the precursor gas maintained at 0% by volume and the RF power source set at -400 Vb. The deposited hydrophobic thin layer had a uniform thickness of about 10 nm and the pressure in the chamber was 5 Pa. The properties of the microporous layer in the hydrophobic porous medium formed in the present example were analyzed, and the following results were obtained.

As in 2 As shown in Figure 1, the surface of the microporous layer formed in Example 1 had a structure in which a hydrophobic thin layer was deposited on a nano-protruded surface. The contact angle on the hydrophobic surface having the above structure is 150 ° or more as in 2D is shown, and the contact angle can be controlled according to the conditions of deposition of the hydrophobic thin film.

The contact angle was measured using a goniometer (Data Physics Instrument GmbH, OCA20L), and this device makes it possible to detect the visual appearance and contact angle of a drop sitting on the surface. The "contact angle" as referred to for simplicity in the present invention indicates the "static contact angle". The contact angle (i.e., the static contact angle) was measured by gently placing a 5 ml drop on the surface.

In 3 showing the contact state of the drop on the surface of the microporous layer, the contact angle on the surface of the microporous layer before etching is about 145 °. However, when the nano-protrusions are formed on the surface and the hydrophobic thin film is deposited thereon, the contact angle increases to about 160 to 170 ° depending on the duration of the oxygen plasma treatment, and therefore, the surface has superhydrophobicity.

As a result, according to the above-mentioned production method, it is possible to provide a highly hydrophobic microporous layer and, since the surface of the microporous layer has a self-cleaning function and a water-repellent function, the porous medium having the above-described microporous layer can be advantageously used as the material can be used, which can maintain the performance of a fuel cell, since efficiently the water generated during the electrochemical reaction of the fuel cell can be removed.

Example 2 Material for the Macroporous Substrate with Increased Hydrophobicity

The macroporous substrate on the other side (see 1 ) of the porous medium has carbon fibers in the form of a felt and PTFE as a porous material distributed between the carbon fibers. Therefore, the contact angle of the macroporous substrate itself with respect to a drop is about 135 to 145 °. Besides, the contact angle of the macroporous substrate is similar to that of the intrinsic microporous layer. The carbon fibers constituting the macroporous substrate are provided in the form of a felt, and the diameter of the carbon fibers is in a range of about 5 to 20 microns.

First, the surface of a generated macroporous substrate was subjected to plasma etching treatment with oxygen using RF-PECVD, and the oxygen plasma etching was conducted under conditions where only oxygen was used as the gas, the etching pressure was 10 Pa, and the RF voltage - 100 Vb to -800 Vb.

The oxygen plasma reacts with the carbon materials by etching the carbon fibers on the surface of the macroporous substrate, thereby bonding the carbon materials to the oxygen plasma to form CO ₂ or CO, thereby etching the surface of the macroporous substrate. The shapes of the surface before and after the plasma etching are corresponding in the 4A and 4B 5, it can be seen that after plasma etching, nano-protrusions having a large aspect ratio are formed from the surface of the micrometer-sized carbon fibers, thereby forming a micro-nano-double structure.

5 FIG. 12 is a graph showing the change in the contact angle of a drop on the surface of the macroporous substrate while changing the duration of plasma etching by means of oxygen. As in 5 For example, when the duration of the plasma etching with oxygen at a voltage of -400 Vb during the plasma etching is changed to 1, 2, 5 to 10 minutes, nano-protrusions on the surface of the carbon fibers and the surface of the PTFE become between the carbon fibers educated.

The projections here have a width of 10 to 30 nanometers and a length of 10 to 200 nanometers, resulting in an aspect ratio of 1 to 7. As a result, the carbon fibers having a diameter of 5 to 20 microns and the nano-protrusions having a high aspect ratio formed thereon form a micro-nano-double structure with protrusions, thereby forming a micro-nano-double (compound) surface. Structure is provided. This surface has superhydrophobic and self-cleaning properties to provide the superhydrophobicity to be achieved with the present invention.

Then, a hydrophobic thin film containing silicon and oxygen for increasing hydrophobicity was deposited on the macroporous substrate comprising the double structure having protrusions of the surface of the carbon fibers having the nano-protrusions thus formed and the PTFE surface. In this method, the hydrophobic thin film was deposited by RF-PECVD at 13.56 MHz of HMDSO while keeping the content of argon gas in the precursor gas at 0% by volume and the RF power source fixedly set to -400 Vb has been. The deposited hydrophobic thin layer had a uniform thickness of about 10 nm and the pressure in the chamber was 5 Pa. The properties of the macroporous substrate formed in the present example in the hydrophobic porous medium were analyzed, and the following results were obtained.

As in 4 As shown in FIG. 2, the surface of the macroporous substrate formed in Example 2 has a structure in which a hydrophobic thin layer is deposited on the surface of carbon fibers having nano-protrusions and the surface of PTFE.

In this structure, the contact angle on the surface of the hydrophobic porous medium is as in 5 is shown, 150 ° or more, and the contact angle can be controlled according to the conditions of plasma etching and according to the conditions of deposition of the hydrophobic thin carbon layer.

The contact angle was measured using the goniometer of Example 1, and this device makes it possible to detect the visual appearance and contact angle of a drop sitting on the surface. The static contact angle was measured by gently placing a 5 ml drop on the surface.

As in 5 is shown, the static contact angle on the surface of the macroporous substrate itself is about 145 °. However, when the nano-protrusions are formed on the surface and the hydrophobic thin carbon layer is deposited on the surface, the macroporous substrate has super-hydrophobicity, which corresponds to a static contact angle of greater than 155 °, with the static contact angle as a function of duration the treatment with oxygen plasma changes.

As a result, according to the manufacturing method described above, it is possible to provide a highly hydrophobic macroporous substrate, and, since the surface of the macroporous substrate has a self-cleaning function and a water-repellent function, the porous medium having the macroporous substrate described above can be advantageously used Material can be used, which can maintain the performance of a fuel cell, since efficiently the water generated during the electrochemical reaction of the fuel cell can be removed.

As such, according to the production method of the present invention, it is possible to substantially increase the hydrophobic properties of the surfaces of the microporous layer and the macroporous substrate to provide a highly hydrophobic porous medium.

In particular, according to the method of the present invention for enhancing the hydrophobic properties of the surfaces of the microporous layer and the macroporous substrate, it is possible to substantially increase the hydrophobic properties of a gas diffusion layer containing the microporous layer and the macroporous substrate.

As described above, the contact angle on the surface of the highly hydrophobic porous medium of the present invention significantly increases, so that the porous medium of the present invention has superhydrophobic surface properties. This superhydrophobic surface has a self-cleaning function and a water-repellent function. Therefore, the highly hydrophobic porous medium of the present invention can be advantageously used as a porous medium for a fuel cell such as a gas diffusion layer and efficiently remove the water generated during the electrochemical reaction of the fuel cell, whereby the high performance of the fuel cell can be maintained.

The invention has been described in detail with reference to preferred embodiments. One skilled in the art will recognize, however, that these embodiments may be varied without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

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Cited non-patent literature

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Claims (20)

Porous medium with increased hydrophobicity, the porous medium comprising a micro-nano-double structure in which nanometer-sized protrusions or collapsed pores are formed on the surface of the porous medium having a surface roughness in the micrometer range; and a hydrophobic thin layer is deposited on the surface of the micro-nano-double structure. The porous medium of claim 1, wherein the porous medium comprises only a macroporous substrate or both a macroporous substrate and a microporous layer coated on the macroporous substrate. A porous medium according to claim 2, wherein the macroporous substrate comprises a micro-nano-double structure in which nano-protrusions or nanopores with a width of 10 to 30 nanometers, a length of 10 to 200 nanometers and an aspect ratio of 1 to 7 on the Surface of carbon fibers are formed with a diameter of 5 to 20 microns. The porous medium of claim 1, wherein the nano-protrusions or nanopores have a width of 1 to 100 nanometers, a length of 1 to 1,000 nanometers, and an aspect ratio of 1 to 10. The porous medium of claim 1, wherein the hydrophobic thin layer is a thin hydrocarbon layer. The porous medium of claim 5, wherein the hydrophobic thin layer is a thin hydrocarbon layer containing silicon and oxygen, or a thin hydrocarbon layer containing fluorine. A porous medium according to claim 1 or 5, wherein the hydrophobic thin layer has a thickness of 0.1 to 90 nanometers. The porous medium according to claim 1, wherein the surface on which the hydrophobic thin layer has been formed has a static contact angle of 150 ° or more. A method of producing a porous medium having increased hydrophobicity, the method comprising: providing a porous medium having a surface roughness in the micrometer range; forming a micro-nano-double structure on the surface of the porous medium by forming nanometer-sized protrusions or collapsed pores by plasma etching; and depositing a hydrophobic thin layer on the surface of the micro-nano-double structure. The method of claim 9, wherein the porous medium comprises only a macroporous substrate or both a macroporous substrate and a microporous layer coated on the macroporous substrate. The method of claim 10, wherein the plasma etching is performed on both sides of the porous medium. The method of claim 9, wherein the plasma etching is a plasma enhanced chemical vapor deposition (PECVD). The method of claim 12, wherein the PECVD is performed using O ₂ , Ar, N ₂ , He, CF ₄ , HF, C ₂ F ₆ , CHF ₃ or SiF ₄ . The method of claim 9, wherein the size and shape of the nano-protrusions or nano-pores during the plasma etching are controlled by adjusting at least one of the irradiation time with plasma, the accelerating voltage and the etching pressure. The method of claim 14, wherein the acceleration voltage is set in a range of -100 Vb to -1000 Vb, and the etching pressure is set in a range of 1 Pa to 10 Pa. The method of claim 9, wherein the plasma etching is performed by ion beam, chemical vapor deposition from hybrid plasma (HPCVD) or atmospheric plasma. The method of claim 9, wherein hexamethyldisiloxane gas is used to deposit the hydrophobic thin film. The method of claim 9, wherein the hydrophobic thin layer is a thin hydrocarbon layer containing silicon and oxygen, or a thin hydrocarbon layer containing fluorine. A method according to claim 9 or 18, wherein the hydrophobic thin layer has a thickness of 0.1 to 90 nanometers. The method of claim 9, wherein the surface on which the hydrophobic thin film is formed has a static contact angle of 150 ° or more.

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